The structure of the last line is like that of "Thy knee bussing the stones" (Cor. iii. 1). "The mind is its own place" (Par. Lost, i. 254), and similar places. There can be little doubt, I think, that on't was effaced at the end of the third line; for the poet could hardly, even in his most careless moment, have termed solid parts of a building 'pendent nests,' etc. Wordsworth, with this very place in his mind, wrote: "On coigns of vantage hang their nests of clay" (Misc. Son. 34). It is also in favour of this reading that it throws the metric accent on this, thereby adding force. 'Coign of vantage' would seem to be coin d'avantage, Fr., and denoting a projection of some kind.
Sc. 7.
"If it were done when 'tis done then 'twere well
It were done quickly. If the assassination
Could trammel up the consequence, and catch
With his success surcease; that but this blow
Might be the be-all and the end-all here;
But here—upon this bank and shoal of time—
We'd jump the life to come.... But in these cases," etc.